using System;

using System.IO;

public class Program

{

    static Random random = new Random();

    public static void Main()

    {

        var exts = new[] { "text", "test" };

        var sourcePath = @"C:\File.txt";

        var release = true;

​

        var pathWithoutExt = Path.GetFileNameWithoutExtension(sourcePath);

        var result = release ? $"{pathWithoutExt}.{exts[random.Next(0, exts.Length)]}" : pathWithoutExt;

​

        Console.WriteLine(result);

    }

}